There is a vitrified grinding tool in which pores are formed by using an organic pore-forming agent. In this kind of vitrified grinding tool, however, there is the problem that retention force of abrasive grains decreases since the pore-forming agent is removed during firing.
On the other hand, in the case of a super abrasive grain grinding tool having a low concentration, it is conventional to admix an inorganic abrasion-resistive material from an economical viewpoint. In this case, however, since the abrasion-resistive material is worn out by abrasion to offer problems to grinding depending upon certain grinding conditions, it becomes necessary to frequently perform conditioning, and advantages as a vitrified super abrasive grain grinding tool cannot sufficiently be utilized under such a situation.
Consequently, there has been disclosed a vitrified grinding tool in which an inorganic hollow substance is used as a material for forming pores, and which has a superporous structure capable of simplifying conditioning without decreasing the retention force of abrasive grains (Japanese Patent Kokai Publication No. 62-251077 (1987). However, although this grinding tool is effective without the range of up to 45-55% by volume for the content of the inorganic hollow material in raw material composition, there is a danger of a decrease in the retention force of abrasive grains and a decrease in the surface finish of a work to be processed, if the content of the inorganic hollow material is increased in order to produce a grinding tool having a further lower concentration or more porous property.